Today, there are hundreds of millions of people in the United States and around the world who wear eyeglasses or contact lenses to correct ocular refractive errors. The most common ocular refractive errors include myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and presbyopia.
Myopic vision can be modified, reduced, or corrected by flattening the cornea axisymmetrically around the visual axis to reduce its refractive power. Hyperopic vision can be modified, reduced, or corrected by steepening the cornea axisymmetrically around the visual axis to increase its refractive power. Regular astigmatic vision can be modified, reduced, or corrected by flattening or steepening the cornea with the correct cylindrical curvatures to compensate for refractive errors along various meridians. Irregular astigmatism often requires correction by a more complex refractive surgical procedure. Presbyopic vision can be modified, reduced, or corrected by rendering the cornea multifocal by changing its shape annularly so that one annular zone focuses distant rays of light properly while another annular zone focuses near rays of light properly.
There are various procedures that have been used to correct ocular refractive errors, such as laser thermal keratoplasty (LTK). LTK uses laser light to heat the cornea, causing portions of the cornea to shrink over time. For example, human corneal stromal collagen may shrink when heated to a temperature above approximately 55° C. The stroma is the central, thickest layer of the cornea. The stroma is formed mainly of keratocytes embedded in an extracellular matrix composed of collagen fibers, proteoglycans, water, and other materials. If the pattern of stromal collagen shrinkage is properly selected, the cornea is reshaped to reduce or eliminate one or more ocular refractive errors. LTK typically does not remove corneal tissue and does not penetrate the cornea physically with a needle or other device.
A problem with LTK and other procedures is regression of refractive correction, meaning the correction induced during a procedure is reduced or eliminated over time and an ocular refractive error returns. Corneal wound healing may be one cause of this regression, and a corneal wound healing response may be triggered by damage to the corneal epithelium in the cornea. The corneal epithelium can be damaged, for example, if it is heated to a temperature of approximately 70° C. or greater, even if only for a period of a few seconds or less.
Moreover, during reshaping of a cornea using LTK or another procedure, it may be desirable to use one or more ocular or ophthalmic solutions, such as a topical anesthetic. However, these ophthalmic solutions may often interfere with the LTK or other procedure, such as by interfering with the corneal reshaping.